(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having at least one air-fuel ratio sensor downstream of a catalyst converter disposed within an exhaust gas passage.
(2) Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio.
According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.X simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration portion of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, if the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio feedback correction amount FAF is also fluctuated, thereby causing fluctuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: U.S. Pat. Nos. 3,939,654, 4,027,477, 4,130,095, 4,235,204). In a double O.sub.2 sensor system, another O.sub.2 sensor is provided downstream of the catalyst converter, and thus an air-fuel ratio control operation is carried out by the downstream-side O.sub.2 sensor is addition to an air-fuel ratio control operation carried out by the upstream-side O.sub.2 sensor. In the double O.sub.2 sensor system, although the downstream-side O.sub.2 sensor has lower response speed characteristics when compared with the upstream-side O.sub.2 sensor, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream-side O.sub.2 sensor, for the following reasons:
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream-side O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalyst converter, these pollutants have little affect on the downstream side O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration of oxygen in the exhaust gas is approximately in an equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstream-side O.sub.2 sensor is compensated for by a feedback control using the output of the downstream-side O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects a deterioration in the emission characteristics. On the other hand, in a double O.sub.2 sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double O.sub.2 sensor system, even if only the output characteristics of the downstream-side O.sub.2 are stable, good emission characteristics are still obtained.
In the above-mentioned double O.sub.2 sensor system, for example, an air-fuel ratio feedback control parameter such as a rich skip amount RSR and/or a lean skip amount RSL is calculated in accordance with the output of the downstream-side O.sub.2 sensor, and an air-fuel ratio correction amount FAF is calculated in accordance with the output of the upstream-side O.sub.2 sensor and the air-fuel ratio feedback control parameter (see: U.S. Pat. No. 4,693,076). In this case, the air-fuel ratio feedback control parameter is stored in a backup random access memory (RAM). Therefore, when the downstream-side O.sub.2 sensor is brought to a non-activation state, such as a fuel cut-off state, to stop the calculation of the air-fuel ratio feedback control parameter by the down- stream-side O.sub.2 sensor, the air-fuel ratio correction amount FAF is calculated in accordance with the output of the upstream-side O.sub.2 sensor and the air-fuel ratio feedback control parameter which was calculated in an activation state of the downstream-side O.sub.2 sensor (i.e., an air-fuel ratio feedback control mode for the downstream-side O.sub.2 sensor) and was stored in the backup RAM. Note that, in a fuel cut-off state, an air-fuel ratio feedback control for the upstream-side O.sub.2 sensor is also prohibited.
In the above-mentioned double O.sub.2 sensor system, the air-fuel ratio feedback control conditions for the downstream side O.sub.2 sensor are as follows:
the coolant temperature is higher than a predetermined value;
the engine is not in an idling state;
the engine is not in a fuel cut-off state;
a secondary air suction system is not driven for forcibly causing the air-fuel ratio upstream of the catalyst converter;
the downstream-side O.sub.2 sensor is in an activation state.
Other conditions may be introduced. Therefore, even when all the air-fuel ratio feedback conditions for the downstream-side O.sub.2 sensor are satisfied, the downstream-side O.sub.2 sensor may be not completely in an activation state or the O.sub.2 storage effect of the three-way catalysts may remain. For example, when the engine is in a fuel cut-off state or in a lean driving state for forcibly causing the engine to be in a lean air-fuel ratio, regardless of the output of the O.sub.2 sensors, the three-way catalysts absorb O.sub.2 molecules, and therefore, immediately after the engine returns to a driving state of the stoichiometric air-fuel ratio, the three-way catalysts expel the stored O.sub.2 molecules therefrom. This is a so-called O.sub.2 storage effect. Particularly, at a descending driving mode, if racing occurs too frequently this invites fuel cut-off operations, and the O.sub.2 storage effect is remarkably exhibited. As a result, even when the air-fuel ratio upstream of the catalyst converter is actually rich, the air-fuel ratio downstream of the catalyst converter is lean for a long time, so that the output of the downstream-side O.sub.2 sensor indicates a lean state. Therefore, if an air-fuel ratio feedback control for the downstream-side O.sub.2 sensor is carried out immediately after the engine is switched to a driving state of the stoichiometric air-fuel ratio, the air-fuel ratio feedback control parameter may be so large or small that an air-fuel ratio feedback control by the upstream-side O.sub.2 sensor using the air-fuel ratio feedback control parameter produces an overrich air-fuel ratio, thus increasing the HC and CO emissions, and raising the fuel consumption. Particularly, in a system where the air-fuel ratio feedback control parameter is stored in the backup RAM in a fuel cut-off state or the like, as explained above, if frequent switching from a fuel cut-off state to a fuel cut-off recovery state and vice versa occurs, the controlled air-fuel ratio becomes further overrich, which means that an air-fuel ratio feedback control for the downstream-side O.sub.2 sensor is meaningless.
The above-mentioned overrich air-fuel ratio problem is true for a single O.sub.2 sensor system having only one O.sub.2 sensor downstream of the catalyst converter.